Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material. Sand casting in most common and most versatile of the casting processes, accounting for over 90% of all metal castings. Molds made of sand (such as silica, zircon, olivine, or chromite sand) are relatively cheap and sufficiently refractory. Sand is mixed with small amounts of other materials, such as clay and water, to develop the strength and plasticity of the clay and to make the aggregate suitable for molding and is then packed around a pattern that has the shape of the desired casting.

If the pattern is to be removed before pouring, the mold is usually made in two or more segments. An opening called a sprue hole is cut from the top of the mold through the sand and connected to a system of channels called runners.The molten metal is poured down the sprue hole, flows through the runners, and enters the mold cavity through one or more openings, called gates. Gravity flow is the most common means of introducing the metal into the mold.The metal is allowed to solidify, and the mold is then broken to permit removal of the finished casting. Because the mold is destroyed in product removal, a new mold must be made for each casting.

Sand molding can be classified further as follows:
 

1. Conventional Sand Molding Process
   1. Green Sand Moulding
   2. Dry Sand Moulding
   3. Flaskless Moulding
2. Chemical Sand Moulding Process
   1. Shell Moulding
   2. Sodium Silicate Moulding
   3. No-Bake Molding

The casting process can be summarized in following six steps.

1. Making of Pattern
2. Molding and core Making
3. Melting and pouring
4. Solidification
5. Cleaning
6. Inspection

A pattern is the first and principal requirement of the casting process. This is the first step to determine the quality of the produced casting which largely depends on the material, design, and construction of the pattern. It is the replica of the object to be made by the casting process, with some modifications. The main modifications are the addition of pattern allowances and the provision of core prints. If the casting is to be hollow, additional patterns called cores are used to create these cavities in the finished product.

Functions of the Pattern

1. A pattern prepares a mold cavity for the purpose of making a casting.
2. A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow.
3. Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern.
4. Patterns properly made and having finished and smooth surfaces reduce casting defects.
5. A properly constructed pattern minimizes the overall cost of the castings.

Pattern Material

Patterns may be constructed from the following materials. Each material has its own advantages, limitations, and field of application. Some materials used for making patterns are:

1. Wood,
2. Metals and alloys,
3. Plastic,
4. Plaster of Paris,
5. Plastic and rubbers,
6. Wax,
7. Resins.

The usual pattern materials are wood, metal, and plastics.

Ideal Characteristics of Pattern Materials

To be suitable for use, the pattern material should be:

1. Easily worked, shaped and joined
2. Light in weight
3. Strong, hard and durable
4. Resistant to wear and abrasion
5. Resistant to corrosion, and to chemical reactions
6. Dimensionally stable and unaffected by variations in temperature and humidity
7. Available at low cost

1. Contraction allowances / Shrinkage allowance
2. Draft allowance
3. Finishing or Machining allowance
4. Shake allowance
5. Distortion allowance

Types of Patterns

The common types of patterns are:

1. Single piece pattern
2. Split piece pattern
3. Loose piece pattern
4. Gated pattern
5. Match pattern
6. Sweep pattern
7. Cope and drag pattern
8. Skeleton pattern
9. Follow board pattern

Sand is the principal molding material and it is used for all types of metal castings, irrespective of the material of cast metal. The most important characteristic of sand is its refractory nature. Ordinary sands (silica (SiO₂), zircon, olivine, or chromite) are mixed with additives to meet four requirements:

Properties of Molding Sand

There are certain properties which molding sand must possess which are listed below:

1. Flowability 
2. Refractoriness
3. Permeability
4. Green Strength
5. Dry Strength
6. Hot Strength
7. Cohesiveness (Bond)
8. Collapsibility
9. Durability 

Composition of molding sands

The primary ingredients of molding sand are:

1. Base Sand
2. Binder
3. Moisture

Classification of Moulding Sand

Molding sand can be classified into three different types:

1. Natural Sands
2. Synthetic Sands
3. Special Sands

Additive to Moulding and Core Making Sand

According to the requirement of molten metal and base sand, suitable additives are added to obtain specific characteristics in molding and core making sands. The commonly used additives are:

1. Coal Dust
2. Iron Oxide
3. Dextrin
4. Molasses
5. Sulphite Lye
6. Linseed Oil (Core Oils)
7. Sodium Silicate (Water Glass)
8. Fibrous Materials

Sand Preparation and Conditioning

To achieve good molding, it is important for each grain of sand to be coated uniformly with the proper amount of additive agents. This is achieved by sand conditioning. Proper sand conditioning and preparation helps as following:
 

• Binder is uniformly distributed around the sand grains
• Moisture is evenly dispersed in the sand mixture
• The sand gets aerated, causing the sand grains to separate and increasing the flowability of sand
• The foreign particles are separated from the sand

Various type of equipment used for sand conditioning and preparation are as following
 

1. Magnetic Separator
2. Riddle
3. Muller or mixer
4. Aerator

Sand Testing

To achieve consistent and high-quality castings standard test and procedures have been developed for the following qualities of molding sand.

1. Grain Size
2. Moisture Content
3. Clay Content
4. Permeability
5. Compressive Strength or Mold Hardness
6. Green Strength or Compactability
7. Shatter index

Other Sands

Though molding sands are prime materials used in a foundry, there is a number of other sand used for obtaining some specific characteristics which are as following:
 

1. Facing Sand
2. Mould Wash
3. Backing Sand
4. Parting sand

The process of making sand molds can be further classified as follows:

1. Conventional Sand Moulding Process
   1. Green Sand Molding
   2. Dry Sand Molding
   3. Skin-Dried Molding
2. Chemical Sand Molding
   1. Shell Molding
   2. Sodium Silicate Molding
   3. No-Bake, Air-set or chemically bonded Molding

Manual Molding Process:

When only a few castings are to be made, manual molding is often the preferred method to make a sand mold. Hand ramming, however, is slow, labor intensive, and usually, results in the non-uniform compaction.

The procedure for making the sand mold manually is as follows:

• The first step in making mould is to place the pattern on the moulding board.
• The drag is placed on the board.
• Dry facing sand is sprinkled over the board and pattern to provide a non-sticky layer.
• Moulding sand is then riddled in to cover the pattern with the fingers; then the drag is completely filled.
• The sand is then firmly packed in the drag by means of hand rammers. The ramming must be proper i.e. it must neither be too hard or soft.
• After the ramming is over, the excess sand is levelled off with a straight bar known as a strike rod.
• With the help of vent rod, vent holes are made in the drag to the full depth of the flask as well as to the pattern to facilitate the removal of gases during pouring and solidification.
• The finished drag flask is now rolled over to the bottom board exposing the pattern.
• Cope half of the pattern is then placed over the drag pattern with the help of locating pins. The cope flask on the drag is located aligning again with the help of pins. 
• The dry parting sand is sprinkled all over the drag and on the pattern.
• A sprue pin for making the sprue passage is located at a small distance from the pattern. Also, riser pin, if required, is placed at an appropriate place.
• The operation of filling, ramming and venting of the cope proceed in the same manner as performed in the drag.
• The sprue and riser pins are removed first and a pouring basin is scooped out at the top to pour the liquid metal.
• Then pattern from the cope and drag is removed and facing sand in the form of paste is applied all over the mould cavity and runners which would give the finished casting a good surface finish.
• The mould is now assembled. The mould now is ready for pouring

Machine Molding Process:

For production work involving a large quantity of production, machine molding is a necessity. Molding usually begins with a pattern and a flask. The mixed sand (mold material) can be packed in the flask by one or more basic ramming techniques, wich are:

1. Jolting
2. Squeezing
3. Sand Slinging

Casting processes are unique in their ability to easily incorporate complex internal cavities or reentrant sections.To produce these features, however, it is often necessary to use cores as part of the mold. Generally speaking, the word "core" means the mass of dry sand that is prepared separately by being baked and then placed in the mold.

In general, cores are surrounded on all sides by the molten metal and are, therefore, subjected to much more severe thermal and mechanical conditions and as a result, the core sand should be of better quality than the molding sand.

Characteristics of Core

• Green Strength
• Dry Strength
• Refractoriness
• Permeability
• Collapsibility
• Low Gas Emission

Ingredients of Core Sand

The ingredients of core sand are sand grains, binders, and other additives to achieve specific properties.

The binders generally used are as following:

• Linseed Oil
• Core Oil
• Cereals
• Resins
• Dextrin
• Molasses
• Sulphite Liquor
• Proteins

Types of Cores

The cores are of two types depending on the type of sand used.

• Green-Sand Cores
• Dry Sand Cores

Dry-sand cores can be made in a number of ways. In each, the sand, mixed with some form of the binder, is packed into a wood or metal core box that contains a cavity of the desired shape. Some methods are as below:

1. Dump-Core BoxProcess
2. Split-Core Box Process
3. Core Oil Process
4. Hot-Box Process
5. Cold-Box Process